The present invention relates to the field of fiber optic data communications, in particular fault detection arrangements for laser safety in a duplex open loop parallel optical link.
Laser-based devices and systems have been used widely in the fields of, for example, communications, computing technology and medical technology. The lasers utilized in these systems have output optical powers that are potentially harmful to both people and equipment. For instance, such lasers are driven at such a power so as to have damaging effects if exposed to a human eye. Among the safety methods and systems that have been developed, Method and Apparatus for Laser Safety described in U.S. Pat. No. 5,999,549 (Freitag, et al.), which is a related patent that is incorporated herein by reference, resets a laser fault counter if a second laser fault condition is not detected within a predetermined reset time period after a laser is turned on.
In the field of fiber optic data communications, fiber optic data communications links must ensure the optical power being transmitted by the laser remains below a defined level, or a xe2x80x9csafexe2x80x9d level, in the event of a single failure in the link so as to avoid the aforementioned potential harm to both people and equipment. The xe2x80x9csafexe2x80x9d level may include, for example, a standard established by industry and/or governmental regulations.
For serial optical links, there are at least two exemplary methods for ensuring that laser-driving, optical power does not exceed the relevant xe2x80x9csafexe2x80x9d level, thus ensuring the safety of the users and any surrounding people as well as preventing any damage to the apparatus by the laser optical power. A first example method includes setting the optical power delivered by the laser to a level that is well below the xe2x80x9csafexe2x80x9d level and utilize circuits on the transmitter IC to detect when the optical power level exceeds the safe level. Since the optical power in serial optical links is most often controlled by a monitor photo diode control loop, the laser average optical power is known. Therefore, fault detection circuits are able to easily determine when the average optical power exceeds a threshold limit. That is, since the current in the monitor photodiode is proportional to the optical power output by the laser, the transmitter can detect when the monitor photodiode current exceeds a threshold limit.
A second example method for ensuring that laser-driving optical power does not exceed the safe level in a serial optical link includes an Open Fiber Control (OFC) handshake protocol. This example protocol is used when the laser-driving optical power in normal data mode is set above the safe level. Thus, when a serial optical link fiber is pulled, according to the OFC protocol, the laser light is pulsed at an extremely low duty cycle (on for approximately 150 psec, off for approximately 10 sec) to ensure that the average laser optical power does not exceed the safe level. Similar to the first method, fault detection circuits on the transmitter side also ensure that a fault in the corresponding laser driving circuit does not cause the optical power to exceed the safe level.
However, such example methods of ensuring that the laser optical power remains at or below a safe level are not applicable to open loop parallel optical links. That is, in open loop parallel optical links, the average optical power is unknown, because there are no monitor photodiodes, and multiple lasers are emitting light simultaneously. Thus, the example fault detection methods described above are inappropriate since the aggregate optical power in an open loop parallel optical link is above the safe level and is much higher than that of a serial link.
For instance, FIG. 2 shows an open loop parallel optical transmitter which includes N+1 channels. The parallel optical transmitter 200 includes global temperature coefficient adjustment DAC (TEMPCO DAC) 210 and global temperature coefficient adjustment shift register (TEMPCO SHIFT REGISTER) 220 which holds bits for the TEMPCO DAC 210. Each of channels 0 through N include a respective laser driver 230, a threshold current adjustment DAC 240, a modulation current adjustment DAC 250, and a shift register 260 to hold the bits for each DAC. EEPROM 270 stores the bits in a non-volatile memory when the parallel transmitter is powered off. This parallel transmitter, however, does not show any method for preventing the aggregate optical power out of the lasers of channels 0 through N from exceeding the xe2x80x9csafexe2x80x9d level.
Accordingly, an object of the present invention is to provide a method and apparatus for ensuring that the laser optical power does not exceed a xe2x80x9csafexe2x80x9d level in an open loop parallel optical link. To that end, the present invention includes a duplex parallel optical link having a transmitter and receiver pair and a fiber optic ribbon that includes a designated number N of channels that cannot be split. The duplex transceiver includes a corresponding transmitter and receiver that are physically attached to each other and cannot be detached therefrom, so as to ensure safe laser optical power.
That is each duplex transceiver includes both a parallel optical transmitter and a parallel optical receiver packaged together along with a fiber ribbon cable where both directions of laser transmission are fixed together. The fiber ribbon cable includes N+1 channels which are bundled together and therefore cannot be split from each other. Channels 0 through N on both the transmitter and receiver sides include one channel which is designated as the safety channel. When the fiber ribbon is connected at both ends of the link, that is between both transceivers, the designated safety channel on both the transmitter and receiver sides function as normal data channels. However, when the fiber ribbon is pulled or is otherwise severed, the signal detectors at the receivers transmit a xe2x80x9closs of signalxe2x80x9d condition to the respective transmitter. The xe2x80x9closs of signalxe2x80x9d signals cause all of channels 0 through N, except for the designated safety channel, of the transmitter to shut down. That is, only the designated safety channel remains enabled.
The optical powers on the designated safety channels are set to a previously determined safe level at manufacturing. However, when the fiber is pulled or otherwise severed, a fault in the laser driver circuit could force a high current into the laser, causing the optical power to exceed the safe level. Fault detection circuits, therefore, must protect the designated safety channels from launching unsafe optical power in the event of a single failure.
The duplex transceivers each include an open loop parallel optical transmitter that includes fault detection circuits that detect a high current condition which causes the optical power to exceed the safe level when the ribbon fiber is pulled or otherwise severed.
In the event that the ribbon fiber is pulled or otherwise severed, since the aggregate power of a parallel link exceeds the safety limit, all of channels 0 through N except for the designated safety channel must be disabled by two mechanisms, which are regarded as being a redundant configuration. That is, if only one mechanism is used to disable the lasers of channels 0 through N, except for the designated safety channel, a single fault in the laser shutdown mechanism would enable the optical power to exceed the safe level. Thus, in a configuration having only one signal detector on the receiver side having its output stuck high, indicating that a signal is present, the aggregate optical power would exceed the limit if the fiber ribbon was pulled or otherwise severed.
The present invention also includes another redundant part of the safety scheme involving the laser current comparison function. A laser fault detection circuit is provided that compares the threshold current and modulation current in the laser of the designated safety channel with a redundant threshold current reference and a redundant modulation current reference. Such redundancy is provided so that, if a high laser current is caused by the circuit that generates the threshold current and modulation current, comparing these values to a current that is also generated from this faulty current generator would cause the optical power in the laser of the designated safety channel to remain at an unsafe level. Therefore, the threshold current and modulation current in the laser of the designated safety channel and the compare currents originate from independent sources.
Yet a further redundant part of the safety scheme involves reading the threshold DAC codes and modulation DAC codes into a shift register from an EEPROM upon power up. That is, if only part of the EEPROM data is read into the shift register, the threshold current or the modulation current could be too large, causing the optical power transmitted in the laser of the designated safety channel to be above the safe level. Thus, the present invention further includes two safety mechanisms, that are implemented to ensure all of the data is read into the shift register. The first method used is to ensure the last bit of the shift register gets loaded with a logic 0. Upon power up, the shift register is set to an all logic 1 state. The first bit out of the EEPROM is a logic 0, which should eventually get shifted to the last bit of the shift register. The second method to ensure that the shift register gets loaded correctly is to count the number of clock cycles required to shift the logic 0 into the last shift register location. The shift register is loaded correctly when the last location in the shift register is a logic zero and the clock cycle count is correct.
Yet another safety consideration provided by the present invention is PFET P1 330. If there is a glitch on the power supply to the transmitter that is low enough to trip the POR circuit, the shift register will be set to an all xe2x80x9c1xe2x80x9ds state. If this power supply glitch does not cause the memory pointer in the EEPROM to reset to memory location 0 (the first bit out must be a logic 0 for the redundant shift register read operation), data will be read into the shift register starting from the wrong memory location, possibly causing the laser currents to be too high in the designated safety channel. These large currents may force the optical power above the safety level. Thus, PEET P1 330 limit disables the power supply to the EEPROM when a power supply glitch occurs on the transmitter.
The fault detector circuit includes a voltage comparator which has an output that goes to a logic xe2x80x9c1xe2x80x9d value if the voltage on the laser of the designated safety channel exceeds a reference voltage. A threshold current comparator forces its output to a logic xe2x80x9c1xe2x80x9d when the threshold current of the designated safety channel exceeds the redundant current threshold. A modulation current comparator forces its output to a logic xe2x80x9c1xe2x80x9d when the modulation current of the designated safety channel exceeds the redundant current threshold. A multiplexer ensures that the voltage comparator functions properly during manufacturing tests. A failing voltage above the reference can be routed to the comparator to force the output thereof to a logic xe2x80x9c1xe2x80x9d value. A laser fault is indicated if the output of any one of the comparators goes to a logic xe2x80x9c1xe2x80x9d value.